1. Field of the Invention
The present invention relates to an apochromat optical system, and more particularly, it relates to an apochromatic lens for a photoengraving process.
2. Description of the Prior Art
It is well known in the art that an apochromatic lens can be formed by combining positive and negative lenses made of glass materials which are different in Abbe's number .nu.from each other and equal or substantially equal in partial dispersion ratio P to each other. According to U.S. Patent Application No. 82-419705 filed on Sept. 20, 1982, for example, a lens group including first and second lenses shown in Table 1 performs a function of apochromat.
TABLE 1 ______________________________________ first lens second lens ______________________________________ name of glass FK52 LaK8 n.sub.d 1.48605 1.71300 .nu..sub.d 81.8 53.8 P.sub.g,F 0.538 0.545 ##STR1## ##STR2## ##STR3## ______________________________________ where symbols n.sub.d, n.sub.g, n.sub.F and n.sub.C denote indexes of refraction with respect to d-lines, g-lines, F-lines and C-lines, respectively, while symbol .nu.d denotes an Abbe's number with respect to the d-lines.
A doublet of the aforementioned apochromatic lens is applied to an objective lens for an astronomical telescope or a collimator lens, in practice.
In the aforementioned combination of the first and second lenses, however, the first lens having a positive power has a low index of refraction and the second lens having a negative power has an index of refraction which is higher than that of the first lens as understood from Table 1, and hence It is difficult to obtain a flat image field. Thus, the aforementioned apochromatic lens cannot be applied to a photographic lens having a wide field angle.
There has been provided a photographic apochromatic lens having a wide field angle, which has the following structure: The photographic apochromatic lens is formed by a stop and first and second lens groups, which include a negative lens made of Kultz flint glass (KzF, KzFS) having an anomalous dispersion value and a positive lens made of glass material being substantially equal in partial dispersion ratio to the Kultz flint glass and higher in index of refraction than the Kultz flint glass, respectively. The first and second lens groups are symmetrically arranged with respect to the stop. For example, optical glass materials for the positive and negative lenses are combined as SK3 and KzF6, SKI6 and KzF1 or LaKN13 and KzFSN4, and partial dispersion ratios P.sub.g,F thereof coincide with each other within a range of 0.54 to 0.56.
In the aforementioned structure, however, the Abbe's number of the glass material forming the positive lenses is approximate to that of the glass material forming the negative lenses, and hence radii of curvature of the respective lenses are reduced as compared with focal length of the apochromatic lens, and an F-number is about F9. In order to cause absolutely no vignetting, it is necessary to set the F-number at about F20. Thus, an apochromatic lens having a large aperture ratio cannot be provided.
In the aforementioned structure, further, a sagittal image field is extremely curved and hence astigmatic difference is increased in a peripheral part of an image. In order to solve this problem, it is necessary to set the F-number at F22 to F32. Thus, the aperture ratio of the apochromatic lens is further reduced.
In order to solve the aforementioned problem, there have been proposed means for increasing the index of refraction of a positive lens by combining positive and negative lenses which are relatively approximate in partial dispersion ratio P.sub.g,F to each other, as shown in Tables 2 to 4.
TABLE 2 ______________________________________ first lens second lens ______________________________________ name of glass LaK08 KzFSN4 n.sub.d 1.69350 1.61340 .nu..sub.d 50.8 44.3 P.sub.g,F 0.555 0.560 ##STR4## ##STR5## ##STR6## ______________________________________
TABLE 3 ______________________________________ first lens second lens ______________________________________ name of glass LaFN2 KzFS5 n.sub.d 1.74400 1.65412 .nu..sub.d 44.8 39.7 P.sub.g,F 0.565 0.570 ##STR7## ##STR8## ##STR9## ______________________________________
TABLE 4 ______________________________________ first lens second lens ______________________________________ name of glass LaSF07 KzFS8 n.sub.d 1.86300 1.72047 .nu..sub.d 41.5 34.7 P.sub.g,F 0.567 0.582 ##STR10## ##STR11## ##STR12## ______________________________________
Also, there have been proposed means for reducing refractive power of each lens by increasing difference in Abbe's number between positive and negative lenses.
As understood from Tables 2 to 4, partial dispersion values (n.sub.F -n.sub.C) and (n.sub.g -n.sub.F) of the positive lens are approximate to those of the negative lens, respectively, and hence the aforementioned combination of the lenses is suitable as that for a photographic apochromatic lens.
In order to effectuate an apochromatic lens, however, it is necessary to keep difference in Abbe's number between the positive and negative lenses within a constant range. Therefore, when the index of refraction of a positive lens is increased, that of Kurtz flint glass (negative lens) must be also increased and hence flatness cannot be improved as compared with the prior art. In other words, flatness is improved by an increase in the index of refraction of the positive lens, but is deteriorated by an increase in the index of refraction of the negative lens.
Since it is necessary that the negative lens be made of Kurtz flint glass for the aforementioned reason, selection of an optical glass material for the positive lens with respect to the negative lens is limited in a constant range. Therefore, the difference in Abbe's number between positive and negative lenses is limited to a constant range. The aforementioned problem cannot be solved, even if the positive and negative lens are selected so that the difference is maximum.